The present invention relates to micro-electro-mechanical systems (MEMS) and in particular to an accelerometer and related microfabrication processes for the high-volume manufacture of such a device.
Presently, micro-structure devices called MEMS (micro-electro-mechanical systems) are gaining popularity in the microelectronics industry. Such MEMS devices include, for example, micro-mechanical filters, pressure micro-sensors, micro-gyroscopes, micro-resonators, actuators, rate sensors, and acceleration sensors. These MEMS devices are created by microfabrication processes and techniques sometimes referred to as micromachining. These processes involve the formation of discrete shapes in a layer of semiconductor material by trenching into the layer with an etch medium. Because MEMS typically require movement of one or more of the formed shapes relative to others, the trenching is done in part over a cavity and in part over a substrate or bonding layer.
MEMS technology can be used to form rotary accelerometers. The main structure of a typical MEMS rotary accelerometer comprises a proof mass supported by a flexure suspension that is compliant for rotation but stiff for translation. In a known device, the suspension comprises fingers extending radially from the body straddled by inwardly projecting capacitor plates mechanically grounded to surrounding annular substrate area; see U.S. Pat. No. 5,251,484, xe2x80x9cROTATIONAL ACCELEROMETERxe2x80x9d issued Oct. 12, 1993 to M. D. Mastache and assigned to Hewlett-Packard Co. of Palo Alto.
Forming the body mass and micro-mechanical parts of the MEMS device can generally be accomplished, for example, by a process of anisotropically etching through one or more upper layers of semiconductor material(s) which are situated above a cavity previously etched into a lower semiconductor substrate. Such a process for forming the body mass and micro-mechanical suspension parts of a MEMS device is often referred to as a xe2x80x9cbond/etch-backxe2x80x9d process. Other processes, however, can instead be utilized to form and/or release the body mass and micro-mechanical parts of a MEMS device. Such other processes can include a through-the-wafer etch process; a lateral release etch (confined or isotropic) process; or a lateral selective undercut etch of a buried layer, a film, or a buried etch-stop layer after a MEMS delineation etch has been performed.
In addition to properly forming the main structures of the MEMS accelerometer, electrically conductive lines are typically integrated with the structure to provide electrical communication between the structure and other microelectronic circuits. See FIG. 1 of the Mastache patent identified above. Furthermore, such a device is typically encapsulated and hermetically sealed within a microshell (i.e., a cap). The microshell serves many purposes, some of which include, for example, shielding the micro-mechanical parts of the MEMS device from particle (such as dust) contamination, shielding the micro-mechanical parts from corrosive environments, shielding the MEMS device from humidity (stiction) and H2O (in either the liquid or vapor phase), shielding the MEMS structure from mechanical damage (such as abrasion), and accommodating the need for the MEMS device to operate in a vacuum, at a particular pressure, or in a particular liquid or gas (such as, for example, dry nitrogen) environment.
A typical MEMS device has a size on the order of less than 10xe2x88x923 meter, and may have feature sizes of 10xe2x88x926 to 10xe2x88x923 meter. This poses a challenge to the structural design and microfabrication processes associated with these small-scale, intricate and precise devices in view of the desire to have fabrication repeatability, fast throughput times, and high product yields from high-volume manufacturing. However, the achievement of these goals often primarily depends upon the ability to successfully execute the critical etching process step in accordance with a desired predetermined shape of the body mass and the micro-mechanical parts of a proposed MEMS device.
MEMS devices such as rotary accelerometers having opposing projections (fingers) which are interdigitated can present a challenge in the microfabrication processes particularly where dimensionally different but equally critical gap spacings must be etched at the same time. This is a result of the fact that wider gaps typically etch faster than narrower gaps.
There is a need in the art for an improved structural design for a MEMS device having interdigitated elements such as projections which will reduce or eliminate the adverse effects associated with the etch process. There is also a need in the art for an improved implementation of the etch process which can be utilized to specifically fabricate the above-mentioned improved structural design for a MEMS device having opposing, interposed and interspaced projections which will circumvent and thereby negate the adverse effects associated with the etch process.
The present invention provides a micro-electro-mechanical sensor structure with an improved design comprising rigid interdigitated projections forming capacitive plate elements and, in a preferred embodiment, flexible projections forming a rotationally compliant suspension. According to the invention, the micro-electro-mechanical structure basically comprises a semi-conductor layer which is micromachined to define a proof mass suspended relative to a support substrate by one or more flexible suspension projections extending from the proof mass to a substrate-based support area. Between these suspension projections and also extending outwardly from the proof mass are sets of additional rigid, spaced apart projections which move with the proof mass according to a compliance mode established by the suspension elements, e.g., at right angles to the longitudinal axes of the finger-like projections. Interdigitated with such projections are complemental projections extending from the support area toward the proof mass and defining, in combination with the rigid body projections, narrow sensor gaps of uniform width and larger, parasitic capacitive gaps. The sensor gaps are formed to exhibit essentially constant gap widths such that the etch process is easily geared to their formation with no loss of accuracy due to different etch rates in other areas of the film.
In the illustrative embodiment, the proof mass is generally circular and the suspension elements and interdigitated capacitance elements are radially arranged. The compliance mode in this embodiment is circular or rotary. However, linear devices using the principles hereafter explained are readily designed.
The present invention further provides an improved process for fabricating the micro-electro-mechanical structure with its improved design for opposing, interdigitated projections consistent with general bond/etch-back methods of fabrication. The process basically includes the steps of providing a first substrate, etching a cavity within the first substrate, and forming an isolation layer on the first substrate. Further steps include providing a second substrate, doping the top portion of the second substrate to thereby form an etch termination layer, forming al doped epitaxial layer on the etch termination layer portion of the second substrate such that the etch termination layer portion of the second substrate has a higher doping concentration than the epitaxial layer. Then, the second substrate is bonded to the first substrate such that the epitaxial layer covers the cavity and is bonded to the isolation layer at the periphery of the cavity of the first substrate. Then, the non-termination layer portion of the second substrate is removed from the etch termination layer portion of the second substrate, and the etch termination layer portion of the second substrate is removed from the epitaxial layer. A photoresist is then applied on the epitaxial layer, and the photoresist is patterned according to a predetermined shape of the micro-electro-mechanical structure. Thereafter, a step of anisotropically etching through sections of the epitaxial layer, as revealed by the patterned photoresist, is performed to thereby define and release the micro-electro-mechanical structure above the cavity,. The remaining patterned photoresist is then removed.
According to a preferred process of the present invention, the step of doping the top portion of the second substrate to thereby form an etch termination layer preferably includes the step of doping the top portion of the second substrate with a p-type dopant comprising boron and germanium. In addition, the step of forming a doped: epitaxial layer preferably includes the step of doping the layer with a p-type dopant. Furthermore, the first substrate and the second substrate preferably comprise silicon, and the isolation layer preferably comprises silicon dioxide.
Also according to the preferred process of the present invention, the step of applying photoresist on the epitaxial layer includes the step of utilizing a positive photoresist. In addition, the step of anisotropically etching through the epitaxial layer to, define and release the micro-electro-mechanical structure above the cavity preferably includes the step of contacting the epitaxial layer with a plasma comprising sulfur hexafluoride and oxygen, and the step of cooling the epitaxial layer to a cryogenic temperature of less than about 173 EK.
Further, according to the preferred process of the present invention, the step of pattering the photoresist according to a predetermined shape preferably includes the steps of determining a minimum capacitive gap between the interdigitated projections of the micro-electro-mechanical structure which are nearest to each other, defining the predetermined shape such that each base of each projection is proximate to at least one tip of another projection by a distance substantially equal to the minimum capacitive gap, and selectively removing the photoresist to reveal bare sections of the epitaxial layer according to the predetermined shape.
Other advantages, structural and process design considerations, and applications of the present invention will become apparent to those skilled in the art when the detailed description of the best mode contemplated for practicing the invention, as set forth hereinbelow, is read in conjunction with the accompanying drawings.